a. Field of the Invention
This invention relates to fluidized bed combustors, and in particular, a means and method for controlling the load turndown ratio in a fluidized bed combustor.
b. Problems in the Art
Combustion of fuel in a fluidized bed is a conventional and well-known process. The advantages of fluidized bed combustion include, but are not limited to, high rate of heat transfer between the hot bed material containing the burning fuel and the wall enclosing the bed, the ability to handle a wide variety of fuel types, and the ability to enact pollution control and treatments.
A significant problem exists, however, with fluidized combustion bed systems. While heat transfer capabilities between the combusting fuel and the wall of the combustion bed are improved by the fluidization material, the ability to control the rate of heat transfer, called the load turndown for the bed, is very limited.
In many cases, the fluidized bed combustor is utilized to produce heat or steam. Depending on the use, it is many times desirable to be able to closely control and vary the combustor output, which is controlled by the heat transfer rate or load turndown. While an obvious method of controlling turndown ratio is to increase or decrease combustion in the fluidized bed, which would produce a corresponding increase or decrease in heat, doing so can have adverse effects on both combustion efficiency and pollution emissions, both of which are primary parameters in most combustion environments.
A variety of methods have been attempted to allow control of load turndown without deleterious effects to combustion efficiency and pollution control. As discussed above, if bed temperature is changed, efficiency is compromised as well as pollution control. Furthermore, temperature change itself is limited by combustion processes themselves.
Another method varies the fluidization air in different sections of the fluidization bed to produce partial bed slumping which in turn reduces active in-bed heat transfer areas. Primary problems with this method are that air leakage to the slumped side causes agglomeration, and splash causes bed material transfer to the slumped side and makes any restarting in that side difficult.
The method of reducing fluidized bed height by reducing the velocity of fluidization air (called "velocity turndown") results in erosion of tubes in the splash zone, and a bed height change is insufficient for large margin turndowns.
Reducing in-bed heat transfer area by actually discharging some of the bed materials also results in erosion of tubes in the splash zone and presents very troublesome particle handling problems.
Independent fluidization or aeration of a separated bed section (called a "wing panel") is insufficient because the heat transfer coefficient is insensitive or too sensitive to such velocity change, and this system is limited to small combustion units.
The prior methods of controlling load turndown in a fluidized bed combustor have many practical difficulties and can provide only partial control of load turndown.
It is therefore a primary object of the present invention to provide a means and method for controlling load turndown in a fluidized bed combustor which improves over or solves the problems and deficiencies in the art.
Another object of the invention is to provide a means and method for controlling load turndown in a fluidized bed combustor which allows a wide variation in load turndown without adverse effects on combustion efficiency and pollution emissions.
A further object of the invention is to provide a means and method for controlling load turndown in a fluidized bed combustor which allows control of the load turndown independent from control of the combustion of the fluidized bed combustor.
Another object of the invention is to provide a means and method for controlling load turndown in a fluidized bed combustor which is simple, efficient, and precisely controllable.
These and other objects, features, and advantages of the invention will become apparent with reference to the accompanying specification and drawings.